Optical communication systems are replacing other communication mediums due to several advantages over conventional systems. For example, optical communication systems typically provide wide bandwidth and low attenuation, immunity to electrical noise, and the ability to securely transmit signals over long distances, including transoceanic links. A typical coherent optical modem processes a received optical communication signal by mixing it with a local-oscillator (LO) signal and then processing the resulting mixed signals to determine the phase and amplitude of the communication signal in each time slot (symbol period), thereby recovering the encoded data. Coherent technology, powered by advanced Digital Signal Processing (DSP), provides access to a rich set of information on the optical field. Yet, for practical reasons, dynamic optimization of coherent modems remains elusive, especially for receivers installed in host environments that render servicing inconvenient if not impossible (e.g., the floor of the Atlantic Ocean).
Instead, modems are conventionally optimized only at initial deployment under start-of-life (SOL) conditions, such as in a laboratory or during network installation and commissioning. During such optimization procedures, various performance margins and operating characteristics, such as end-of-life (EOL) Quality factor (Q-factor or Q) or optical signal-to-noise ratio (OSNR) margin, can be measured and optimized through conventional optical noise loading experiments. On an in-service cable system, however, this approach can disrupt traffic bearing channels and may unacceptably degrade network operation as a whole. This procedure is also inherently prone to error and is becoming more difficult as advances in digital coherent technology allow for improved spectral efficiency and dual-polarization modulation, such that noise floors can no longer be measured.
Current practices in performance budgeting as well as system acceptance are therefore unable to take advantage of the real-world, dynamic set of performance measures offered by coherent technology, despite the inherent limitations and continually diminishing effectiveness of current optimization procedures. For example, network aging and repair activities tend to degrade performance over time. Changes in the environment where the modem is installed may, from time to time, also affect performance. Current practices either ignore these effects or overcompensate for them with overly cautious performance margins. More often, the latter is chosen, with negative impact on system cost.
Accordingly, there is in general a need for more effective techniques for dynamic optimization of optical modem performance, and in particular, for techniques to directly measure various operating characteristics and available performance margins without having to resort to conventional optical noise loading.